1. Field
Embodiments of the present invention relate to a method and apparatus for determining scattering coefficients and analyte concentrations.
2. Related Art
Optical spectroscopy and spatially resolved measurements of light propagated through and/or reflected off of turbid media have been widely used to determine concentration of analytes, scattering coefficients, reduced scattering coefficients, and other physiologically relevant parameters in biological samples and tissues. These methods are typically based on measuring the propagation of light modulated by intrinsic optical properties of biological tissues and/or samples. Known optical properties of biological samples and tissues have been used to assess parameters related to metabolism, function of various tissues and organ systems, etc.
Spatially resolved diffuse spectroscopy is used to monitor clinical states of a patient or user by computing concentrations of analytes and scattering coefficients using spatially and spectrally resolved measurements of propagated light. Multivariate calibration techniques and diffusion model-based approaches are used to determine concentrations of analytes such as hemoglobin derivatives and other commonly observed analytes.
Non-uniqueness is a disadvantage of the measurement techniques mentioned above. That is, several sets of optical properties, such as several different scattering coefficients or reduced scattering coefficients, could correspond to numerically identical measurements of reflectance. At a certain range of emitter-detector separations (lateral distances between a light emitter and detector), at least two values of reduced scattering coefficients at a specified value of an absorption coefficient may correspond with numerically identical measurements of reflectance. These factors can result in large errors in quantification of analyte concentrations. One way of addressing this problem is to constrain scattering coefficients or reduced scattering coefficients to values available from published literature to determine concentrations from multiple wavelength reflectance or transmittance measurements.
Additionally, another disadvantage of the prior art methods is undesired cross-talk between absorption and scattering parameters, which has been demonstrated in a publication by Corlu et al. A. Corlu, T. Durduran, R. Choe, M. Schweiger, E M. C. Hillman, S. R. Arridge, and A. G. Yodh, entitled “Uniqueness and wavelength optimization in continuous-wave multispectral diffuse optical tomography,” Opt. Lett. 28, 2339-2341 (2003).
Various prior art publications disclose analyzing reflectance and absorption of propagated light in determining analyte concentrations. Both reflectance and absorption are components of propagated light that take into account frequency and time components along with multipath interference and scattering factors, such as scattering coefficients. Scattering coefficients have largely been ignored or avoided in prior art methods.
Simple and complex calibration approaches have been developed to characterize tissues based on their scattering coefficient or absorption alone or some combination thereof. Additionally, modified diffusion theory and Monte-Carlo based expressions have also been developed and utilized for this purpose. Scattering coefficient and absorption related parameters such as hemoglobin derivatives and other analyte concentrations have been estimated using these methods. A limitation of these methods is that they are restricted to the UV-VIS region of the spectrum where the absorption of analytes is significantly large. The penetration depth at these wavelengths is very shallow when compared to wavelengths in the NIR regions (650-1000 nm). Another limitation of prior art methods using the diffusion model or diffusion theory is that this typically results in more unknowns than equations.
Accordingly, there is a need for a method and apparatus for determining analyte concentrations that overcomes the limitations of the prior art.